Green synthesis of graphene by using tomato juice

ABSTRACT

Embodiments of the present invention encompass methods of forming graphene from graphene oxide and/or graphite oxide using tomato juice.

FIELD

This invention relates to materials science, and more specifically,methods of manufacturing graphene from graphene oxide.

BACKGROUND

Allotropes, which are different physical forms of the same element, ofcarbon include diamond, graphite, and charcoal. Graphite is a layeredstructure of stacked two-dimensional planes of a honeycomb or hexagonallattice formed by sp² hybridized carbon atoms. In 2004, these individualplanar layers of carbon atoms, graphene, were isolated. In addition toforming graphite, graphene forms the basis of fullerenes, and carbonnanotubes. Graphene exhibits unique mechanical, thermal, and electricalproperties.

Methods exist to form graphene from graphene oxide. Some chemicalmethods involve toxic chemicals, and some intermediates produced bythese chemical methods may be explosive. Thermal methods exist, butrequire a large use of energy, which is expensive.

Thus, additional methods of production of graphene from graphene oxideare needed.

INCORPORATION BY REFERENCE

All publications, patents, and patent applications mentioned in thisspecification are herein incorporated by reference to the same extent asif each individual publication, patent, or patent application wasspecifically and individually indicated to be incorporated by reference,and as if each said individual publication, patent, or patentapplication was fully set forth, including any figures, herein. To theExtent that any definition provided herein conflicts with a definitionprovided in any of the incorporated by reference materials, thedefinition provided herein controls.

SUMMARY

Embodiments of the present invention encompass methods of forminggraphene from graphene oxide. The methods include forming a suspensioncomprising graphene oxide, graphite oxide, or both, and a solvent, or inother words, forming a suspension comprising the product of oxidation ofgraphite and a solvent. Fresh tomato juice is added to the suspension.The graphite, graphite oxide, or both are exfoliated. After exfoliation,such as by sonication of the suspension before, after, during, or acombination thereof the addition of the tomato juice, the suspensionsits quiescently for a time period to form graphene. After thesuspension has reacted, the solids are separated from the suspension.The initial graphite oxide, graphene oxide, or both may be the productof an oxidation reaction on graphite.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts the X-ray diffraction (XRD) pattern of graphite.

FIG. 2 depicts the X-ray diffraction (XRD) pattern of graphene oxide.

FIG. 3 depicts the X-ray diffraction (XRD) pattern of a graphene oxidesample produced by an embodiment of the present invention.

FIG. 4 depicts the Raman spectra of graphite.

FIG. 5 depicts the Raman spectra of graphene oxide.

FIG. 6 depicts the Raman spectra of a graphene oxide sample produced byan embodiment of the present invention.

DETAILED DESCRIPTION

The term “as used herein” applies to the entire disclosure and expresslyincludes the specification, claims, and drawings.

As used herein, the use of the singular includes the plural and viceversa unless expressly stated to be otherwise. That is, “a,” “an” and“the” refer to one or more of whatever the word modifies. For example,“an article” may refer to one articles, two articles, etc. By the sametoken, words such as, without limitation, “articles” would refer to onearticle as well as to a plurality of articles unless it is expresslystated or obvious from the context that such is not intended.

As used herein, words of approximation such as, without limitation,“about,” “substantially,” “essentially,” and “approximately” mean thatthe word or phrase modified by the term need not be exactly that whichis written but may vary from that written description to some extent.The extent to which the description may vary from the literal meaning ofwhat is written, that is the absolute or perfect form, will depend onhow great a change can be instituted and have one of ordinary skill inthe art recognize the modified version as still having the properties,characteristics and capabilities of the modified word or phrase. Withthe preceding discussion in mind, a numerical value herein that ismodified by a word of approximation may vary from the stated value by±15% in some embodiments, by ±10% in some embodiments, by ±5% in someembodiments, or in some embodiments, may be within the 95% confidenceinterval.

As used herein, any ranges presented are inclusive of the end-points.For example, “a temperature between 10° C. and 30° C.” or “a temperaturefrom 10° C. to 30° C.” includes 10° C. and 30° C., as well as anytemperature in between. In addition, throughout this disclosure, variousaspects of this invention may be presented in a range format. Thedescription in range format is merely for convenience and brevity andshould not be construed as an inflexible limitation on the scope of theinvention. Accordingly, the description of a range should be consideredto have specifically disclosed all the possible subranges as well asindividual numerical values within that range. As an example, adescription of a range such as from 1 to 6 should be considered to havespecifically disclosed subranges such as from 1 to 3, from 1 to 4, from1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well asindividual numbers within that range, for example, 1, 2, 3, 4, 5, and 6.Unless expressly indicated, or from the context clearly limited tointegers, a description of a range such as from 1 to 6 should beconsidered to have specifically disclosed subranges 1.5 to 5.5, etc.,and individual values such as 3.25, etc. that is non-integer individualvalues and ranges beginning with, ending with or both beginning with andending with a non-integer value(s). This applies regardless of thebreadth of the range.

As used herein, a range may be expressed as from “about” one particularvalue and/or to “about” another particular value, for example from about5 to about 10. When such a range is expressed, another embodiment isincluded, the embodiment being from one particular value and/or to theother particular value. Similarly when values are expressed asapproximations by use of the antecedent “about,” it will be understoodthat the particular value forms another embodiment. As an example, thedisclosure of the range “about 5 to about 10” also discloses the “5 to10.” Similarly, the disclosure of “a temperature of about 100° C.” isequivalent to disclosing “a temperature of 100° C.”

As used herein, “solvent” is defined as a substance capable ofdissolving one or more other substances, capable of at least partiallydissolving the other substance(s), or at least dispersing one or moreother substances to form a uniformly dispersed solution at a selectedtemperature and pressure. A solvent can refer to one chemical compound,or a mixture of chemical compounds. A solvent can be a fluid.

As used herein, the phrase “wt %” means a percent (%) by weight.

Graphene has a wide range of industrial applications including, withoutlimitation, use in forming composites with different types of polymers,and use in sensors, nano-electronics, super capacitors, and batteries.Embodiments of the present invention encompass methods of forminggraphene from graphene oxide using tomato juice, which is a naturalmaterial. Embodiments of the present invention include methods offorming graphene from graphene oxide (or the product of an oxidationreaction on graphite) that are simple, low cost, safe, and environmentalfriendly.

As used herein, the terms “graphene” and “graphene oxide” encompass bothsingle- and multi-layer graphene and graphene oxide, respectively. Asdescribed above, a single layer of carbon atoms or a sheet in thelayered stack of sheets that form graphite is graphene. As used herein,graphene will encompass a single plane or layer of sp²-hybridized carbonatoms, as well as stacks of more than one layer on top of each other,but not more than 10 layers stacked together. The same definitions alsoapply to single- and multi-layer graphene oxide. Graphene and grapheneoxide, like most crystals, may contain impurities and/or defects such asand without limitation, gaps in the carbon lattice, substitution of anon-carbon atom for a carbon atom, or both. Embodiments of the presentinvention encompass methods in which graphene oxide is reduced withtomato juice. Graphene oxide may be obtained by oxidizing graphite tographite oxide and exfoliating the graphite oxide, and/or partially orcompletely exfoliating the graphite during the oxidation process. Theexfoliation may occur before, during, or after the oxidation, and insome cases, before, or during the reduction of the graphene oxide,graphite oxide, or combination thereof, to graphene. The purity of thestarting graphite, graphite oxide, or graphene oxide will depend uponthe ultimate use of the graphene as the properties of the graphene areimpacted by defects.

One method of producing graphitic oxide is described in U.S. Pat. No.2,798,878, issued on Jul. 9, 1957, to W.S. Hummers, Jr. Modified Hummersmethods are described in N. I. Kovtyukhova, P. J. Ollivier, B. R.Martin, T. E. Mallouk, S. A. Chizhik, E. V. Buzaneva, and A. D.Gorchinskiy. “Layer-by-Layer Assembly of Ultrathin Composite Films fromMicron-Sized Graphite Oxide Sheets and Polycations,” Chemistry ofMaterials, ACS publications 11 (1999) 771-778, and Longzhen Zheng, DanYe, Leyan Xiong, Jingpeng Xu, Kun Tao, Zhijun Zou, Danlin Huang, XiaoweiKang, Shaoming Yang, and Jian Xia. “Preparation ofcobalt-tetraphenylpolphyrin/reduced graphene oxide nano composite andits application on hydrogen peroxide biosensor,” Analytical ChemicalActa, 768 (2013): 69-75.” Graphite oxide may also be produced by thermalmethods. In some cases, the product of oxidation of graphite includesgraphene oxide. In some embodiments, the product of the oxidation ofgraphite is a combination of graphite oxide and graphene oxide andpossible unreacted graphite. In some embodiments, dispersing agents areadded to the solution in which the oxidation of graphite occurs.

The graphene oxide, graphite oxide, or combination thereof is suspendedin a solvent. In some embodiments, the phrase “graphene oxide, graphiteoxide, or a combination thereof” also encompasses the product of theoxidation of graphite which may include unreacted graphite andpotentially graphene in addition to graphene oxide and graphite oxide.Water is the preferred solvent. In some embodiments the water isdeionized, and in some embodiments the water is distilled. In someembodiments, the water is used “as received,” without the addition ofother substances (also known as “free of additives”), and withoutfiltering or other processing of the water. In some embodiments, the pHof the water is adjusted. In some embodiments the solvent may be up to50 wt % of a second solvent in combination with distilled water. In someembodiments, the solvent is non-aqueous. In some embodiments, the secondsolvent is an alcohol such as without limitation methanol, ethanol,propanol, or a combination thereof. In some embodiments, there is 0.01wt % to 25 wt % graphene oxide, graphite oxide, or combination thereofdispersed in the solvent, preferably 0.1 wt % to 15 wt % graphene oxide,graphite oxide, or combination thereof dispersed in the solvent, andmore preferably, 0.2 wt % to 5 wt % graphene oxide, graphite oxide, orcombination thereof dispersed in the solvent. In some embodiments, thereis 0.1 wt % to 3 wt % graphene oxide, graphite oxide, or combinationthereof dispersed in the solvent. In some embodiments, the solvent andthe graphene oxide, graphite oxide, or combination thereof comprises atleast 95 wt %, at least 97 wt %, at least 98 wt %, at least 98.5 wt %,at least 99 wt %, or at least 99.5 wt % of the suspension. In someembodiments, the suspension includes only the solvent and the grapheneoxide, the graphitic oxide, or combination thereof.

After the suspension is made, the reducing agent, in this case, freshtomato juice is added. The tomato is the fruit of the plant Solanumlycopersicum. In preferred embodiments, the tomato is ripe. The juicemay be produced by any known method such as without limitation, a handblender, a food processor, or any available juicer for home orcommercial use. In some embodiments, the juice is filtered or strainedto remove solids. The filter or strainer may have approximately squareopenings of 80 micrometers to 3.175 mm on each side. In someembodiments, the juice is used without the addition of any additivessuch as sugar. In some embodiments, the juice is not diluted withadditional water, and not diluted with any other solvent. In preferredembodiments, the tomato juice used is undiluted and has no additives.

The tomato juice used is fresh. In some embodiments, fresh is defined asused within 1 hour of being produced (squeezed from the fruit). In someembodiments, fresh is defined as used within 45 minutes of beingproduced, within 30 minutes of being produced, within 15 minutes ofbeing produced, and within 5 minutes of being produced.

In some embodiments, the weight: weight ratio (or mass:mass ratio) ofthe fresh tomato juice to the sum of the weight of the graphene oxideand graphite oxide ranges from 10:1 to 1:10, preferably 5:1 to 1:5, andeven more preferably 3:1 to 1:3. In some embodiments, the weight (ormass) of the fresh tomato juice to the sum of the weight (or mass) ofthe sum of the graphene oxide and graphite oxide ranges from 1:2 to 2:1.In some embodiments, the weight (or mass) of the fresh tomato juice tothe weight (or mass) of the product of oxidation of graphite ranges from1:2 to 2:1.

In some embodiments, 1 ml of tomato juice is added per each 1-10milliliters of a dispersion the dispersion being 0.1 wt % to 3 wt %graphene oxide, graphite oxide, or combination thereof dispersed in asolvent. In some embodiments, 1 ml of tomato juice is added per each 2-8milliliters of a dispersion, the dispersion being 0.1 wt % to 3 wt %graphene oxide, graphite oxide, or combination thereof dispersed in asolvent. As a non-limiting example, 5 milliliters of tomato juice may beadded to 20 milliliters of an aqueous dispersion of 0.5 wt % grapheneoxide, graphite oxide, or combination thereof.

The graphite oxide/graphene oxide is exfoliated. In other words, thestacked sheets of graphite/graphite oxide are separated. Means ofexfoliation include, without limitation, mechanical means, thermal means(such as, without limitation, use of a temperature of 1100° C.), use ofsupercritical carbon dioxide, addition of water, acid, or base, a pHchange, centrifugation, and sonication. The methods may be usedindividually or in combination, and the use in combination may beconcurrently, separately, or both. Multiple applications may be used, asfor example addition of water, acid, or base, followed by centrifugationon more than one occasion. The exfoliation may occur before, during, orafter oxidation of graphite, before, during, and/or after reduction ofgraphite oxide, graphene oxide, or a combination thereof to graphene. Insome embodiments, the Hummers method of oxidizing graphite produces acombination of graphene oxide and graphite oxide.

In the embodiments of the present invention, sonication is the preferredmethod of exfoliation. In some embodiments, the sonication is executedat less than 20 kHz, and in some embodiments, the sonication is executedat about 20 kHz. In preferred embodiments, ultrasonication (greater than20 kHz) is used. Sonication may occur for a time period ranging from 5minutes to 6 hours, preferably 15 minutes to 3 hours, more preferably 20minutes to 2 hours, and most preferably 30 minutes to 1 hour. In someembodiments, the sonication is executed for 45±5 minutes.

As noted above, the exfoliation may occur before, during, and/or afteroxidation of graphite, and/or before, during, or after reduction ofgraphite oxide, graphene oxide, or a combination thereof, to graphene.In some embodiments, the conditions of oxidation of graphite to graphiteoxide also at least partially exfoliates the graphite/graphite oxidesuch that a mixture of graphite oxide and graphene oxide exists,potentially also including un-oxidized or only partially oxidizedgraphite. In some embodiments, the suspension of graphene oxide,graphite oxide, or combination thereof is sonicated for some time periodbefore the addition of the reducing agent, which is tomato juice in theembodiments of the present invention. In some embodiments, the tomatojuice is added to the suspension of graphene oxide, graphite oxide, orcombination thereof before any sonication. In some embodiments, it is acombination of the above, that is the suspension is sonicated bothbefore and after addition of the tomato juice. In some embodiments, thesuspension may be sonicated during the addition of the tomato juice. Insome embodiments, the suspension is sonicated during the addition of thetomato juice in addition to sonication before and/or after the additionof the tomato juice. In preferred embodiments, the suspension issubjected to ultrasonication at least after the tomato juice has beenadded to the suspension of the graphene oxide, graphite oxide, orcombination thereof.

The sonicated suspension is then allowed to react for a time period. Inembodiments of the present invention, the sonicated suspension isallowed to sit quiescently in an environment at a temperature of in therange of about 18-25° C. In some embodiments, the sonicated suspensionis allowed to sit quiescently in an environment at a temperature of inthe range of about 20-22° C. In some embodiments, the temperature of theenvironment in which the sonicated suspension sits is tightly controlled(±2° C., ±1.5° C., or ±1° C.). In some embodiments, the temperature ofthe environment in which the sonicated suspension sits is not tightlycontrolled (±5° C., with excursions up to 30° C., and/or as low as 15°C., where excursion is a duration of 5 minutes or fewer). In someembodiments, for the environment in which the sonicated suspension sitsthere is no control of pressure, and no vacuum is applied. In someembodiments, the environment in which the sonicated suspension sits isat standard atmospheric pressure, or about 101.325 kPa (±10.1325 kPa)where kPa is kilopascal, or 29.92 inches (inHg) (±2.99 inHg) or 760millimeters of mercury (mmHg) (±76 mmHg). In some embodiments, thehumidity of the environment in which the sonicated suspension sitsranges from 3% rh to 95% rh (rh=relative humidity). In some embodiments,the environment in which the sonicated suspension sits is exposed to airwithout any addition or removal of specific gas. In other embodiments,the environment in which the sonicated suspension sits has a low oxygenlevel (such as note more than 5 volume %, preferably, not more than 2volume %, and most preferably, not more than 1 volume %).

The sonicated suspension with added tomato juice sits quiescently for atime period. Thus, “sits quiescently” means that there is no stirring oragitation of the suspension by any means. The sonicated suspension isnot shaken, rotated, or centrifuged. The sonicated suspension sitswithout external agitation.

The time period during which the sonicated suspension with added tomatojuice sits quiescently ranges from about 12 hours to about 72 hours. Insome embodiments, the time period during which the sonicated suspensionwith added tomato juice sits quiescently is not less than 18 hours, andnot more than 48 hours, preferably, not less than 20 hours, and not morethan 36 hours, and more preferably, not less than 22 hours, and not morethan 30 hours. In some embodiments, the time period during which thesonicated suspension with added tomato juice sits quiescently is 24±1.5hours. After the sonicated suspension with added tomato juice sitsquiescently for the time period, the solids are separated from thesolvent, and then allowed to dry by being placed in an environment at atemperature range of about 18-25° C. In some embodiments, the solids areseparated by using a vacuum filter, and/or a BÜckner funnel with asintered glass disc of 4 to 5.5 micro meters. In some embodiments, thesolids are removed by centrifuging before filtering.

As used herein, with respect to the solids recovered from the suspensionafter the reduction process (the solids recovered from the sonicatedsuspension after the time period of sitting quiescently), “dry” is asolvent content of not more than 5 wt %, and in some embodiments, notmore than 2 wt %. In some embodiments, “dry” is a solvent content of notmore than 1 wt %, and in some embodiments, not more than 0.5 wt %. Inpreferred embodiments, the solvent is water. In some embodiments, thesolids are allowed to dry in an environment at a temperature of in therange of about 20-22° C. In some embodiments, the temperature of theenvironment in which the solids are allowed to dry is tightly controlled(±2° C., ±1.5° C., or ±1° C.). In some embodiments, the temperature ofthe environment in which the solids are allowed to dry is not tightlycontrolled (±5° C., with excursions of up to 30° C., and/or as low as15° C., where excursion is a duration of 5 minutes or fewer). In someembodiments, the environment in which the solids are allowed to dry isat standard atmospheric pressure, or about 101.325 kPa (±10.1325 kPa) or29.92 inches (inHg) (±2.99 inHg) or 760 millimeters of mercury (mmHg)(±76 mmHg). In some embodiments, the environment in which the solids areallowed to dry is under a vacuum (at least 20% lower than standardatmospheric pressure at the specific elevation). In some embodiments,the humidity of the environment in which the solids are allowed to dryranges from 0.1% rh to 95% rh, preferably from 0.1% rh to 50% rh, whererh is the relative humidity. In some embodiments, air or another fluidis blown over the solids to increase the rate of drying.

The product, the graphene solid, has an average diameter of about 50 nmto about 100 nm, such as and without limitation, around 74.7 nm, whichmay be determined from direct measurement of a transmission electronmicroscope (TEM) image. In some embodiments, the average diameter isfrom about 50 nm to about 100 nm as measured using laser-lightscattering or photon correlation spectroscopy.

EXAMPLES

The examples presented in this section are provided by way ofillustration of the current invention only and are not intended nor arethey to be construed as limiting the scope of this invention in anymanner whatsoever.

The following abbreviations may be used in the Examples:

g=grams;

ml=milliliters (10⁻³ liter);

min=minutes;

hr=hour;

hrs=hours;

M=molarity;

kV=kiloVolts;

mA=milliAmpheres;

cm=centimeter, 10⁻² meter;

Å=Ångström, 10⁻¹⁰ meter; and

micron=micrometer, 10⁻⁶ meter.

Example 1

1A. Preparation of Graphene Oxide (GO)

Graphene oxide, graphite oxide, or a combination thereof (GO) wasprepared from natural graphite flakes with size of 50 microns by amodified Hummer's method (see Kovtyukhova et al.; Zheng et al.).Briefly, graphite (3.0 g) was added to concentrated (18.4 M) H₂SO₄ (70mL) under stirring at room temperature (approximately 20-25° C.). ThenNaNO₃ (1.5 g) was added and the mixture was cooled to 0° C. Undervigorous agitation, MnO₄ (9.0 g) was added slowly to keep thetemperature of the suspension lower than 20° C. and the temperature wasmeasured during the process with a thermometer. The mixture was stirredat 35° C. for 2 hours. Then distilled water (150 mL) was added and thesolution was stirred 90° C. for 15 min. An additional 500 ml ofdistilled water was added and followed by a slow addition of 15 mL ofH₂0₂ (3 volume % aqueous solution), turning the color of the solutionfrom dark brown to yellow. The mixture was filtered by vacuum on a largeBÜchner funnel which contained a sintered glass disc, and washed with1:10 HCl (hydrochloric acid) to water aqueous solution (250 mL) toremove metal ions followed by washing with 200 mL of distilled water toremove the acid. The resulting solid was allowed to dry (to a lowerwater content, approximately 5 wt % or lower than 5 wt %) in air in anenvironment at a temperature of about 20° C. to about 25° C., andsubsequently diluted to make a graphene oxide (GO) aqueous dispersion(0.5 wt %). The resulting product GO has a purity of 73.15% of carbonand oxygen of 26.31%.

1B. Reduction of Graphene Oxide by using Fresh Tomato Juice

Fresh juice was prepared from the fruit (tomato) by using a handblender. No sugar or any other substances were added to the tomatojuice. Fresh tomato juice (free of additives, and un-diluted), about 5ml, was added to the GO suspension, about 20 ml of about 0.5 wt %aqueous dispersion, and this suspension was ultrasonicated for 45minutes. The mixture was kept in an environment at room temperature(approximately 20-25° C.) for 1 day (24 hours±3 hours). After 1 day thesuspension colour was changed from brown to black which confirms thatgraphene oxide was reduced by the juice. The solids in the resultingsuspension were filtered out, and subsequently washed and dried at roomtemperature (approximately 20-25° C.). X-ray Photoelectron Spectroscopy(XPS) analysis revealed that the graphene produced in the above processcontained 98% carbon and 1.32% oxygen.

In summary, the above process is:

Example 2

Characterization of Graphite, Graphene Oxide and Graphene.

2.1. XRD Results

The X-ray Diffraction (XRD) patterns of graphite, graphene oxide andgraphene were analyzed by using powder X-Ray diffractometer,specifically Rigaku Miniflex™ 2 Desktop. The XRD patterns were recordedwith Cu radiation at 30 KV, 15 mA in the range of 2θ5° to 70°. Acontinuous scan was used and 2°/min was used as scan speed. The resultsare shown in FIGS. 1 and 2, for graphite and graphene oxide,respectively. FIG. 3 shows the XRD pattern of graphene, which wasprepared by using fresh tomato juice (using a method such as thatdescribed in Example 1).

The XRD pattern of graphite in (FIG. 1) shows a strong sharp diffractionpeak at 2θ of 26.56° which corresponds to an interlayer spacing of 3.35Å. The XRD spectra of Graphene oxide (FIG. 2) shows a peak at 2θ of10.82°, which corresponds to an interlayer spacing of 8.17 Å. The largerinterlayer spacing of 8.17θ is indirect evidence of the oxidationbecause it is significantly larger than that of the graphite. Theincrease in spacing is due to the formation of oxygen contactingfunctional groups between the graphite layers. In FIG. 2, another peakappeared at 2θ of 25.92° which corresponds to an interlayer spacing of3.434 θ.

After reduction of GO with the fresh tomato juice, the peak of 2θ at10.82° disappears and a broad peak appears, starting from 2θ at 22-24°,as exhibited in FIG. 3. This disappearance of the peak is evidence ofthe reduction of GO to the graphene. The presence of the broad peak at22-24° in graphene indicates the presence of few layers of graphene.This peak corresponds to 002 plane of graphite with interlayer spacingof 3.65 Å which is due to the removal of oxygen atoms that got into thegraphite gallery during the intercalation process. These XRD results,shown in FIG. 3, confirm the reduction process of GO to graphene byusing the tomato juice

2.2 Raman Spectroscopy Results

Raman Spectroscopy has become a powerful method to characterize thegraphene and graphene based materials. Raman spectra of graphite andgraphene oxide are shown in FIGS. 4 and 5. Significant structuralchanges happen during the processing from pristine graphite to graphiteoxide, graphene oxide, or a combination thereof. The Raman spectrum ofthe pristine graphite (with 50 micron in size), as expected, displays aprominent G peak as the only feature at 1577 cm⁻¹, corresponding to thefirst-order scattering of the E2g mode. In the Raman spectrum ofgraphene oxide, the G band is broadened and shifted to 1604 cm⁻¹. Inaddition, the D band at 1364 cm⁻¹ becomes prominent, indicating thereduction in the size of the in-plane sp² domains, possibly due to theextensive oxidation. FIG. 6 shows the Raman spectra of a graphene sampleproduced with an embodiment of the present invention by using the tomatojuice. The Raman spectrum of the graphene has both G and D bands as seenin FIG. 6, at approximately at 1578 cm⁻¹ and 1348.3 cm⁻¹, respectively.In addition, as shown in FIG. 6, a new band at 2700 cm⁻¹ is seen in FIG.6, and this new band is referred to as the 2D band and it is seen onlyin the graphene structures. The ratio of I_(2D)/I_(G) of the intensitydepends on the number of graphene layers. The intensity ratioI_(2D)/I_(G)<1 in graphene prepared by using an embodiment of thepresent invention using the tomato juice indicates multiple layers ofgraphene.

Accordingly, it is understood that the above description of the presentinvention is susceptible to considerable modifications, changes andadaptations by those skilled in the art, and that such modifications,changes and adaptations are intended to be considered within the scopeof the present invention, which is set forth by the appended claims.

What is claimed is:
 1. A method comprising: forming a suspensioncomprising graphene oxide, graphite oxide, or both, and a solvent;adding fresh tomato juice to the suspension; exfoliating the graphite,graphite oxide, or both; after exfoliation, allowing the suspension tosit quiescently for a time period at a temperature in the range of about18 °C to about 25 °C to allow the graphene oxide, graphite oxide, orboth to be at least partially reduced to graphene; after the timeperiod, separating the at least partially reduced graphene oxide, atleast partially reduced graphite oxide, or both from the suspension. 2.The method of claim 1, wherein the exfoliation comprisesultrasonication.
 3. The method of claim 2, wherein the suspension withadded tomato juice is subjected to ultrasonication.
 4. The method ofclaim 2, wherein ultrasonication is executed for at least 45 minutes. 5.The method of claim 1, wherein the solvent comprises water.
 6. Themethod of claim 1, wherein 99 wt % of the suspension consists of waterand graphene oxide, graphite oxide, or a combination thereof.
 7. Themethod of claim 1, wherein about 0.1 wt % to about 3.0 wt % of thesuspension comprises graphene oxide, graphite oxide, or a combinationthereof.
 8. The method of claim 1, wherein the fresh tomato juice isadded to the suspension within 45 minutes of the juice being squeezedfrom the tomato.
 9. The method of claim 1, wherein the fresh tomatojuice is free of additives.
 10. The method of claim 1, wherein the freshtomato juice is not diluted with other liquids before being added to thesuspension.
 11. The method of claim 1, wherein the weight ratio of theamount fresh tomato juice added to the amount of the suspension is about2:1 to about 1:2.
 12. The method of claim 1, wherein the time period ismore than 22 hours, and not more than 30 hours.